1. Field of the Invention
The present invention relates to highly pure octafluorocyclopentene, useful as a dry-etching gas for use in forming a very large scale integrated circuit (hereinafter referred to as “VLSI”) pattern or an ultra large scale integrated circuit (hereinafter referred to as “ULSI”) pattern, and a preparation method thereof. More particularly, the present invention relates to a dry etching gas containing octafluorocyclopentene (C5F8) in an amount of 99.995 vol % or greater, nitrogen gas in an amount of 50 vol ppm or less, oxygen gas in an amount of 5 vol ppm or less, water in an amount of 5 wt ppm or less, and metal ingredients in a total amount of 5 wt ppb or less, and a method for preparing the dry-etching gas in a continuous manner with octachlorocyclopentene serving as a starting material.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Dry etching, which refers to the removal of material, typically a masked pattern of semiconductor material, is regarded as an essential process for the fabrication of ULSI, which requires ultra-fine patterns of circuits for storage of a great quantity of information within a small space. When a dry etching process is applied to a layer of silicon oxide, which is typical of silicon compounds, conventional etching gas, that is, saturated fluorocarbon gas, cannot ensure patterning circuits to a fineness of 0.13 μm or less (an aspect ratio of 20 or greater). Saturated fluorocarbons exhibit low selectivity between a silicon oxide compound to be etched and a protecting film, e.g., photoresist or polysilicon, so that they are difficult to apply for etching ultra-fine patterns. After etching, in addition, carbon residues of the conventional etching gas are not completely removed, impeding the formation of ultra-fine patterns.
As an alternative etching gas to conventional saturated fluorocarbons, octafluorocyclopentene (hereinafter referred to as “C5F8”), which contains one double bond, has attracted intensive attention. Thanks to fewer fluorine atoms per carbon atom resulting from the double bond, this alternative etching gas can more selectively etch mask films, such as photoresists, polysilicon, etc., than saturated fluorocarbons. In addition, post-etching residues of the etching gas are readily evaporated, which is helpful for the formation of circuit patterns having a fineness of 0.13 μm or smaller (an aspect ratio of 20 or higher).
C5F8 is a matrix material having a boiling point of 26.8° C.
With the tendency of semiconductor devices toward high integration and performance, etching gas, such as C5F8, for use in the formation of semiconductor elements, is required to be purer. High purity is a condition essential in order for etching gas to conduct etching at a high rate and uniformly. The biggest problem in achieving a highly pure etching gas is residual trace components, which are typically metal. A level of metal ingredients higher than a critical level is not only fatal to the formation of fine patterns, but also has a negative influence on the performance of the semiconductor device. Accordingly, etching gas is most strictly controlled during the fabrication procedure of semiconductor devices as well as during the production and purification thereof. It has recently been required to reduce the level of metal ingredients to less than 5 ppb.
Methods of producing highly pure C5F8 have already been suggested.
Japanese Patent Laid-Open Publication No. Hei. 9-95418 discloses a method of preparing C5F8 at 99.8-99.98% purity by reacting 1,2-dichlorohexafluorocyclopentene with KF in DMF under a stream of nitrogen.
In International Patent Publication No. WO 2000/71497 (PCT/JP2000/03308) is disclosed a gas for plasma reaction, characterized in that the gas has a content of octafluorocyclopentene of 99.9 vol % or more, and the total amount of nitrogen and oxygen contained as residual trace gas components is 200 vol ppm or less. It can be produced by rectifying crude C5F8 from a purity of 95 vol % to a purity of 99.9 vol % or more in an atmosphere of an inert gas belonging to Group 0, and subsequently removing residual impurities therefrom through low-temperature vacuum deareation, molecular screening, or absorbent contact.
In typical method of producing C5F8, octachlorocyclopentene (C5Cl8) or hexachlorocyclopentadiene (C5Cl6) is hydrofluorinated with hydrogen fluoride (HF) in the presence of an antimony (Sb) or chrome (Cr) catalyst to partially substitute the chlorine atoms with fluorine atoms to afford chlorofluorocyclopentenes (C5ClxF8−x, x=1-7) (U.S. Pat. No. 6,395,940), followed by further fluorinating the chlorofluorocyclopentenes with potassium fluoride (KF) in N, N-dimethylformamide (hereinafter referred to as “DMF”) to substitute the chlorine atoms linked to the double bonds with fluorine atoms (Japanese Pat. Laid-Open Publication No. 9-95458).
Chlorine atoms linked to double-bonded carbon atoms are hard to substitute with fluorine atoms using hydrogen fluoride in the presence of an antimony or chromic catalyst. The fluorination of chlorine atoms linked to double-bonded carbon atoms is, accordingly, achieved using potassium fluoride, which requires an additional process step. For fluorination with hydrogen fluoride in the presence of an antimony or chrome catalyst, the preparation, activation and regeneration of the catalyst is needed. In addition, the fluorination is accompanied by complicated processes, including the recovery of excess hydrogen fluoride added, treatment of excess hydrogen chloride produced, the absorption of hydrogen chloride in water and in the course of separation between the product and the hydrogen chloride, and the dehydration of the product.
The direct fluorination of octachlorocyclopentene with potassium fluoride has previously been known (J. Org. Chem. 28 112 (1962)). However, most commercial processes do not take the direct fluorination method using potassium fluoride, but are conducted by partially fluorinating octachlorocyclopentene to chlorofluorocyclopentene in the presence of an antimony or chrome catalyst and subsequently converting chlorofluorocyclopentene to octafluorocyclopentene. The reason for avoiding the direct fluorination of octachlorocyclopentene with potassium fluoride is that it is difficult to maintain a continuous process.
The chemical industry, a kind of process industry, is economically favorable in terms of quality control, manpower, and production cost when the processes thereof are conducted in a continuous manner.
1) Difficulty of Conducting Continuous Process
The direct fluorination of octachlorocyclopentene requires a large quantity of solid potassium fluoride (as much as 8 equivalents or more), compared to the required amount for fluorination of partially fluorinated chlorofluorocyclopentene (CClxF8−x, x=1-3). After the reaction is terminated upon the completion of addition of octachlorocyclopentene, a large quantity of the by-product solid potassium chloride (KCl) is drained, from the reactor, together with the solvent DMF, followed by feeding fresh DMF and potassium fluoride into the reactor and raising the temperature in order to prepare a new round of the reaction. Accordingly, the processes must be conducted in a non-continuous manner in order to remove the large quantity of solid KCl that accumulates in the reactor.
2) Treatment of Potassium Fluoride and Potassium Chloride
When chlorofluorocyclopentene (CClxF8−x, x=1-3) is fluorinated with potassium fluoride in DMF, the amount of potassium fluoride is reduced to ⅛ to ⅜ of the amount required for the fluorination of octachlorocyclopentene. Accordingly, it is relatively easy to treat solid potassium fluoride and potassium chloride. In contrast, the fluorination of octachlorocyclopentene results in the deposition of a large amount of potassium chloride in DMF, thus making it very difficult to treat the potassium fluoride and potassium chloride. Although conducted with the aid of a stirrer, the treatment of the solid (KCl) is not easily solved. Thus, the direct fluorination of octachlorocyclopentene is difficult to apply in practice.
3) Separation of Potassium Chloride from DMF
After the reaction, potassium chloride and DMF are drained from the lower portion of the reactor and separated using a filter so as that the DMF can be reused. The amount of potassium chloride produced is too large to be filtered completely, and it is cumbersome to return the eluted DMF back into the reactor.
International Patent Publication No. WO 2000/71497 discloses a process of preparing C5F8 to a purity of 99.97 vol % by placing 99.83% pure C5F8, along with a boiling chip, in a glass flask equipped with a rectification column, purging the rectification column with He gas, and fluxing the reactants within the flask (Example 1), and a process of further rectifying the C5F8 to a purity of 99.98% in a helium atmosphere (Example 2).
Generally, crude C5F8, which needs to be purified to be used in the semiconductor field, contains organic components including starting materials, intermediates, and by-products, in addition to water, nitrogen, oxygen and metal ingredients as impurities. It is very difficult to remove the organic components from the crude C5F8 since organics having boiling points lower and higher than that of C5F8 coexisting in the organic components.
According to the method of WO 2000/71497, organic materials having boiling points lower than that of C5F8 are removed using an inert gas belonging to Group 0 to give C5F8 with a purity of 99.9 vol % or higher while organics higher in boiling point than C5F8 are filtered using molecular screening or absorbed to an absorbent to yield C5F8 with a purity of 99.9 vol % or higher.
Nowhere in the method of the patent are the contents of metal components mentioned because the crude C5F8 used already has a purity of as high as 95 vol % and the purification process is conducted using glass instruments.
In order to apply the method for the production of C5F8 on an industrial scale, however, on-line analysis is required in real time. In practice, on-line analysis in real time requires the use of metallic reactors and pipes, such as those made from stainless steel, considering the joints between pipes and reaction conditions such as pressure and heat. Accordingly, the products inevitably contain various metal components because they are introduced from the metallic apparatus.
Therefore, the purification method using glass apparatus alone cannot be industrially applied in practice.
Gas for use in fabricating semiconductor devices must be ultra pure. Trace analysis is important in examining the purity of gas. Particularly, C5F8 gas, which exists as a liquid at room temperature, readily incorporates nitrogen and/or oxygen thereinto from the air. In order to maintain the reliability and accuracy of semiconductor products by not overlooking the incorporation of impurities from the air, the on-line analysis of such etching gas must be conducted in real time in a purification stage or an application stage. Since, according to the C5F8 purification method of the international patent (supra), which features the use of glass apparatus and an inert gas belonging to Group 0, the distillation, intake, storage and analysis must be conducted in an incomplete airtight condition, the incorporation of nitrogen and/or oxygen into the product C5F8 is inevitable. In addition, since purified C5F8 is stored, along with the inert gas of Group 0, in a pressure-resistant container, the inert gas occupies the upper portion of the container. It is therefore difficult to discharge only C5F8 at a fixed rate in an early stage of semiconductor process or analysis. Indeed, a large volume of C5F8 gas is discarded prior to semiconductor processes or analysis. Products purified using a gas of Group 0 (He) are very inconvenient for users to treat because the gas discharge is a prerequisite for accurate analysis or reliable semiconductor processes.